Sodium cyanide (NaCN) has a variety of uses. For example, it is used in electroplating, treating metal surfaces, extracting and recovering precious metals from ores, and a number of other chemical applications. The use of NaCN in the leaching of ores that contain precious metals, such as gold and silver, is well known in the art.
To produce NaCN, hydrogen cyanide (HCN) can be neutralized with aqueous sodium hydroxide (NaOH) solution, followed by evaporative crystallization to produce slurry of NaCN crystals, which can be separated from the slurry and dried. During the crystallization of NaCN from the aqueous solution, some liquid can be trapped inside the crystals thereby producing an undesirable NaCN. Also, the aqueous solution can contain certain impurities such as, for example, iron. The impurities decrease the crystal quality. Additionally, HCN can undergo polymerization during the NaCN process to produce HCN polymer, which can, for example, foul equipment and degrade NaCN quality. The NaCN crystals are usually formed by dry compression methods into briquettes.
The briquettes are generally shipped to consumers in containers designed to exclude exposure to atmospheric air for NaCN is very hygroscopic and can absorb substantial quantities of water when exposed to atmospheric air. If exposed to atmospheric air, serious difficulties in shipping and storage can result due to caking. Also, there is the added cost of excluding atmospheric air. Majority of consumers generally convert the NaCN briquettes into an aqueous solution, sometimes after breaking the briquettes into smaller particles, to produce a solution containing about 20 to 25 weight % NaCN. To avoid hydrolysis with the resulting evolution of hazardous hydrogen cyanide vapors, an alkali such as NaOH is added to the dilution water raising the resulting solution pH to about 12 or higher.
To avoid difficulties and cost associated with storage of the anhydrous product and industrial hygiene hazards due to generation of respirable dust when handling anhydrous NaCN briquettes or breaking the briquettes into smaller particles, some consumers have changed to direct solution storage by dissolution of the briquettes in the shipping container, usually tank trucks or railway cars, or in a storage tank, and unloading the resulting solution into storage tanks. Though shipment of NaCN solution is an alternative, it presents high shipping costs and a high environmental risk of spills in the event of an accident during transportation.
Accordingly, developing a process for producing NaCN that eliminates the defects disclosed above would be a great contribution to the art.